Boron Isotopic Composition Of Tourmaline Research Articles

Chemical and boron isotopic compositions of tourmaline from the pegmatite in Ke'eryin rare metal orefield, Eastern Tibet: Implications for pegmatitic evolution

Pegmatites occur widely in the Ke'eryin rare metal orefield. The genesis and evolution of the Ke'eryin pegmatites are still in dispute, and morphological and geochemical studies on tourmalines from the Ke'eryin pegmatites are limited. In this study, in-situ analyses of major and trace elements and B isotope were conducted to uncover the origin of tourmaline and the evolution of related pegmatites. Three types of tourmaline from the Ke'eryin barren pegmatite were identified: elongated columnar or needle-columnar tourmaline (Tur-1 type), isolated, disseminated, irregular, and massive tourmaline (Tur-2 type), and long columnar tourmaline (Tur-3 type). Petrographically, the Tur-1 type crystallized at the early stage of pegmatitic crystallization sequence, the Tur-2 type possibly formed at the early- and/or syn-pegmatitic crystallization sequence, whereas the Tur-3 type likely formed at the relatively late crystallization sequence. Compositionally, most tourmalines belong to the alkali group with a few falling in the vacancy group. All the tourmalines show a schorl composition and are of magmatic origin. Chemical variations from the Tur-1 to Tur-3 tourmalines are controlled by magma fractionation and melt compositions rather than crystal chemical effects. Most tourmalines follow the (Na, Mg)(Xvac, Al)−1, (Mg, OH)(Al, O)−1, and (Ca, Mg2)(Xvac, Al2)−1 exchange vectors. The higher contents of Zn, Sn, Li, Be, Nb, and Ta and negative Eu anomalies in the Tur-3 type indicate that it was likely crystallized at a more evolved stage. In combination with textural evidence and tourmaline chemistry, we suggest that the Tur-1 and Tur-2 types were formed at the relatively earlier stage of pegmatite-forming magma and the Tur-3 type was likely formed closer to the end-stage of barren pegmatite crystallization. The B isotopic compositions are relatively homogeneous and display slightly higher in the Tur-3 type, which were likely caused by fractional crystallization during B-rich magma evolution. It can be inferred that the tourmaline in more evolved and/or Li-mineralized pegmatite with magma evolution should have higher Li, Be, Nb, Ta, and Sn contents, implying that tourmaline chemistry may be used as a potential exploration indicator for rare metal mineralization.

Origin of fluids in the Lancangjiang tin belt, southwestern Yunnan Province, China: Evidence from trace element and boron isotopic compositions of tourmaline

The Lancangjiang tin belt is located on the eastern margin of the Tibetan Plateau, which is in the northern part of the Southeast Asian tin belt that contains Sn, W, and base metal ore deposits. Tourmaline alteration and high B contents are typical features of the magmatic–hydrothermal systems in the Lancangjiang tin belt. Our new data for tourmaline intergrown with cassiterite reveal complex geochemical features that provide important insights into the origins of ore-forming fluids in this B-rich tin belt. Most of the tourmaline in the Sn deposits and metamorphic rocks belongs to the alkali-group dravite series, except for some that is part of the X-vacant group. The tourmaline compositions differ among the ore districts, but all exhibit Fe-poor (0.61–1.08 apfu) and Mg-rich (1.43–2.06 apfu) compositions, with Fe/(Fe + Mg) = 0.18–0.46. Most trace elements in the tourmaline occur at low contents (<50 ppm), including the large-ion lithophile elements (e.g., Cs and Ba) and high-field-strength elements (e.g., Nb, Ta, Hf, and Th). However, some trace elements (e.g., Zn, Sr, and V) and Sn have high contents (up to several hundreds or thousands of ppm). The δ11B values of tourmaline from the Sn deposits range from −14.7 ‰ to −11.3 ‰, except for those in the Man Makhsan Sn deposit, which range from −12.0 ‰ to −8.1 ‰. The δ11B values (−14.7 ‰ to −8.1 ‰) and the positive correlation between Sn contents and Nb/Ta, Al/Ga, and K/Cs ratios in the ore-related tourmaline indicate that the exsolved ore-forming fluids were derived from highly fractionated S − type granitic magmas. The fluids were mainly B-, Sn-, and Al-enriched. The low Fe/(Mg + Fe) ratios, high oxygen fugacity, and Al saturation were favorable for cassiterite precipitation, and are proxies for the ore-forming potential of large-scale Sn deposits.

Chemical and boron isotopic composition of tourmaline from the Yixingzhai gold deposit, North China Craton: Proxies for ore fluids evolution and mineral exploration

Chemical and boron isotopic composition of tourmaline from the Yixingzhai gold deposit, North China Craton: Proxies for ore fluids evolution and mineral exploration

Genesis of the Xianghualing tin-polymetallic deposit in southern Hunan, South China: Constraints from chemical and boron isotopic compositions of tourmaline

The Xianghualing skarn Sn polymetallic deposit (Nanling Range), with ca. 0.25 Mt Sn metal, 0.85 Mt Pb + Zn metal, and 22 Kt (Ta, Nb)2O5, is a Late Jurassic biotite granite-related Sn-Pb-Zn-Nb-Ta deposit. However, our understanding of how magmatic-hydrothermal processes contributed to its mineralization remains poor. Tourmaline has the capacity to retain primary elemental and isotopic information throughout the tin-related ore-formation process, allowing us to use the in-situ major, trace elemental, and boron isotopic compositions of tourmaline to research the origin of tin-bearing fluids and to trace the magmatic-hydrothermal evolution associated with tin mineralization. Five types of tourmaline are recognized based on petrography: i) late-magmatic tourmaline (Tur-I) disseminated in the biotite granites; ii) hydrothermal tourmaline (Tur-II) in massive schorl-fluorite association from the endoskarn zone; iii) hydrothermal tourmaline (Tur-III) with long columnar crystals in massive schorl-epidote association from the exoskarn zone, is related to Sn-Pb-Zn mineralization; iv) hydrothermal tourmaline (Tur-IV) from tourmaline-fluorite veins in exoskarn within the marble; and v) hydrothermal tourmaline (Tur-V) from cassiterite-sulfide-tourmaline-fluorite-topaz veins hosted by quartz sandstone, and is directly related to Sn-Pb-Zn mineralization.Tourmaline from the Xianghualing tin-polymetallic deposit dominates the alkali group and the schorl-dravite solid-solution series. Distinct Fe/(Fe + Mg) and Na/(Na + Ca) ratios of Tur-II to Tur-IV reveal different degrees of B-rich fluid-rock (granite or carbonate) interaction. Trace element variations in tourmaline are principally governed by local fluid-rock interactions and melt/fluid compositions in a low fO2 environment. Late-magmatic tourmaline (Tur-I) and hydrothermal tourmaline (Tur-V) related to vein-type Sn-Pb-Zn mineralization have similar Ca, Na, Al, Mg, Fe, and REE + Y contents, indicating that magmatic-hydrothermal exsolution controls Sn-Pb-Zn enrichment. This process occurs in a reducing environment without significant water–rock reactions, demonstrating that high fO2 or acid-consuming reactions are unnecessary for cassiterite precipitation. Variations in water–rock reactions between magmatic fluids and the carbonates cause differences in tourmaline elemental concentrations (e.g., Ca, Al, and Pb) between the magmatic and skarn phases. There are two main paths of tin mineralization in the Xianghualing deposit: (1) fluorine- and boron-rich fluid exsolution at the magmatic-hydrothermal transition stage is crucial for releasing tin and forming cassiterite during the direct cooling process; (2) fluid-rock interaction between tin-bearing magmatic fluids and carbonates facilitates the precipitation of cassiterite. The small variation in δ11B values for all types of tourmaline (-14.8 to −11.6 ‰) suggests that B-rich fluids are mainly derived from magmatic differentiation and fluid exsolution, with insignificant B isotopic fractionation. Magmatic-hydrothermal evolution at the Xianghualing Sn-polymetallic deposit can be unraveled by using the element and B-isotope fingerprints of tourmaline, which might be used as a prospecting aid for Sn-polymetallic deposits.

Magma contamination in Himalayan leucogranite by metapelite host rocks: insights from chemical and boron isotopic compositions of tourmaline

ABSTRACT Contamination is a common scenario of intracrustal magmatic processes that may significantly change the compositions of involved anatectic melts. Here, we present major element and B isotope analyses on tourmalines from the Guowu leucogranite and their host rocks, which allow us to unravel the potential contaminations of the Himalayan leucogranites by metapelite host rocks. Three types of tourmaline have been identified in our samples. Tur-M in the host schist has the highest Mg/(Mg+Fe) ratio (0.52–0.62) and low Al content, suggesting its metamorphic character. Tourmalines in the leucogranite (Tur-L) and in the contact zone between leucogranite and schist (Tur-C) have an identical core-rim texture, and their cores are characterized by the lowest Mg/(Mg+Fe) ratio (0.13–0.18) and high Al content, which are consistent with the composition of magmatic tourmaline in the Himalayan leucogranites. The rims of Tur-L show higher Mg/(Mg + Fe) ratio (0.32–0.45) and moderate Ca, Ti, F contents, reflecting the contribution of the host schist via contamination. The composition of Tur-C rims is similar to that of Tur-M, suggesting a more significant contribution from contaminated schist than that for Tur-L. The different types of tourmaline share consistent B isotope compositions with δ11B ranging from −14 to −12‰. The compositional characteristics of the contaminated tourmaline and the mineral assemblage in the contaminated zone suggest that the involved components derived from the host schist, probably produced by the decomposition of tourmaline, biotite, plagioclase, primarily include Ca, K, Mg, Ti, B and F. In addition, contamination by tourmaline-bearing metapelite may be an alternative interpretation responsible for the peraluminous character of the Himalayan leucogranites. The occurrence of contaminated tourmaline in leucogranites suggests that contamination of host rocks is a possible way promoting the formation of tourmaline in the Himalayan leucogranites, and tourmaline is useful for deciphering the related contamination processes.

Tourmaline as a recorder of the magmatic-hydrothermal evolution in the formation of pegmatite: In-situ elemental and boron isotopic compositions of tourmaline from the Qinghe pegmatite, Chinese Altay orogen

• Tourmalines record the magma-hydrothermal evolution of pegmatite-forming magma . • Extremely low REEs of tourmaline indicate direct melting of metapelite to form pegmatite . • Boron isotopic variation reflects the scale of fluid exsolution during pegmatitic evolution. We conducted elemental and boron isotope studies on tourmalines from a single pegmatite dyke of the Altay orogen to investigate its origin and evolution. The pegmatite dyke is composed of border, intermediate and core zones. The border and intermediate zone tourmalines mostly belong to schorl, while the core zone and magmatic hydrothermal tourmalines to dravite. Most tourmalines follow the (Na + Mg) (Al + X vac ) -1 , FeMg -1 and MnMg -1 exchange vectors. The core zone tourmalines show positively correlated FeO t and MgO, and negatively correlated FeO t and Al 2 O 3 , suggesting a rise of f O2 in this zone. Moreover, they show higher B, Mg, Sr and Eu than other tourmalines due to magmatic evolution. Tourmaline from the Altay pegmatite is characterized by extremely low REE abundances (<3 ppm) when compared with those from granites, suggesting an origin from direct melting of metapelitic rocks. The border and intermediate zone tourmalines have comparable boron isotopic compositions with δ 11 B = −14.0 to −12.2‰, which are similar to the mica schist (−13.2 ‰ to −12.2‰), but slightly heavier than the core zone and magmatic hydrothermal tourmalines (−14.4‰ to −13.3‰). This suggests derivation of the pegmatite from melting of the metapelite without causing boron isotope fractionation, whereas late-stage fluid exsolution caused somewhat boron fractionation. This may be indicative of only small amounts of fluids being released from the barren pegmatite-forming magma. Tourmalines from mineralized pegmatites basically exhibit heavier boron isotope compositions than those from barren pegmatites, due probably to more involvement of evaporate in the source of the former.

Chemical and boron isotopic composition of tourmaline from the Gonghe Triassic intermediate-acid intrusive rocks, Qinghai and its implications for evolution of the magmatic-hydrothermal system

青海共和盆地干热岩经历的热历史过程、热源是了解干热岩地热藏形成亟待解决的难题, 不同时期、不同类型的脉体可为其经历的热过程、热源提供证据。经调查发现, 盆地东北当家寺岩体及井下干热岩中电气石脉体与该区后期断裂产状相近, 是否代表后期热事件需要确定。本研究选择对GR1井、DR3井中酸性侵入岩岩芯和当家寺露头岩体中发现的电气石脉体开展了岩相学、锆石年代学、电子探针、LA-MC-ICPMS原位微量元素及B同位素分析, 以约束电气石脉体的成因和源区。结果表明, GR1、DR3井岩芯及当家寺岩体含有电气石脉体的岩性分别是碱长花岗岩、高镁闪长岩及二长花岗岩; 其中露头区花岗岩体中电气石脉的宽度约20cm, 产状直立, 其围岩的形成时代为239~241Ma。背散射及显微图像特征揭示, GR1井和DR3井下中酸性侵入岩及当家寺岩体中电气石为碱族的黑电气石和镁电气石, 具有远近不同的多个流体来源。<i>δ</i>¹¹B分布在-11.50‰~-11.93‰, 与大陆地壳平均的同位素组成<i>δ</i>¹¹B值(-10‰±3‰)相近。结合区域地质资料, 认为在晚三叠世时期, 该区域整体处于碰撞期或后碰撞期, 陆壳加厚发生部分熔融形成S型花岗岩(~220Ma), 其中含硼的热液流体侵位于早期具有俯冲背景的I型花岗岩(~240Ma)中形成电气石脉。

Metamorphic evolution of the juvenile Serrinha forearc basin in the southern Brasiliano Orogen

Understanding the accretion of an oceanic island arc to a craton is a major issue in unravelling the evolution of the continents. P-T-t conditions registered in metamorphic rocks, the boron isotopic compositions of tourmaline provide essential information to delimit the accretionary processes. The Tonian-Cryogenian São Gabriel Terrane is a prominent juvenile structure in the southern Brasiliano Orogen that records key information about accretionary processes. Limited previous studies have investigated the São Gabriel Terrane, with little presently known about the metamorphic conditions. To pursue the objective, we combined multiple methods, including pseudosection modeling, U-Pb garnet geochronology and δ11B isotopes of tourmaline applied to one key sample from the Serrinha Formation of the São Gabriel Terrane. The recorded peak P-T conditions occurred at 4.5–5.1 kbar and 536–555 °C. U-Pb dating of low-U (<1 ppm) garnet yielded a lower intercept age of 721 ± 14 Ma for the bulk garnet growth. Dravite isotopic δ11B (+0.51 to + 3.39) compositions indicate oceanic crust components present in the island arc rocks. The calculated P-T conditions correspond to the metamorphic peak from heat pulses during burial events. Collision and thrusting of the São Gabriel arc against and over the Rio de La Plata Craton occurred at 721 ± 14 Ma. The evaluation of the parameters obtained in this study indicate a Barrovian-type evolution and a Late Tonian age of metamorphism for the Serrinha Basin. The sedimentation of the Serrinha Basin occurred in a forearc basin of the São Gabriel island arc. Collision of the oceanic arc with the craton is thus better constrained. Our study contains one point in the evolution of the protracted, complex amalgamation of the supercontinent Gondwana during the Brasiliano Orogeny.

Ore formation of the Dayakou emerald deposit (Southwest China) constrained by chemical and boron isotopic composition of tourmaline

The Cretaceous Dayakou emerald deposit (southwest China) hosted in the Neoproterozoic schists and granofels is a magmatic-hydrothermal ore system. In this paper, we present the petrological and geochemical study of tourmaline from the Dayakou deposit, aiming to constrain the source of ore-forming fluid and emerald genesis. Two types of tourmaline, namely type-I formed in emerald-bearing quartz veins and type-II in non-mineralized quartz veins, are distinguished and analyzed in situ for their chemical and boron isotopic compositions. All tourmalines are close to the dravite endmember and belong to the alkali group. The type-I tourmaline has a higher Fe/(Fe + Mg) ratio (0.28–0.47) with Li enrichment (21–227 ppm). The type-II tourmaline, however, shows a lower Fe/(Fe + Mg) ratio (0.14–0.34) with relatively higher Co (2–3 ppm) and REEs (2–21 ppm) content. Boron isotopic composition in tourmaline (δ11B = −15.8‰ to −10.4‰) is closed to the average boron composition of continental crust. The boron isotope variations could be the result of rapid precipitation of tourmaline from a magmatic-hydrothermal fluid or the mixing with a metamorphic fluid. Based on the chemical composition and enrichment in granitophile elements (e.g., Li, Be, Sn) and transition metals (e.g., Ti, V, Cr) in tourmaline, we propose that the Dayakou deposit is dominantly controlled by the fluid-rock interaction between the metamorphic basement and possibly reduced magmatic-hydrothermal fluids. Beryllium is transported as fluoride complex in fluids. Hydrothermal fluids continuously interacted with surrounding rocks to extract Cr, V, and Ca into fluids, resulting in extensive crystallization of fluorine minerals (e.g., fluorite, fluorapatite, etc.) and subsequent destabilization of Be-F complexes. The large amounts of Be, Cr and V enriched in the fluid then combine to form emeralds. The Li, Co, and REEs contents of tourmaline can be used as the exploration indicator to target emerald mineralization in this region.

Chemical and boron isotopic compositions of tourmaline at the Baiyanghe Be[sbnd]U deposit, northwest China: Implications for Be[sbnd]U mineralization

Tourmaline records the physical and chemical conditions of the crystallization environment and preserves the information through younger magmatic-hydrothermal processes. Therefore, tourmaline is a favorable geochemical tracer and can be used to reveal processes that have contributed to the metallogenesis of hydrothermal deposits. The Baiyanghe deposit, located in the Xiemisitai Mountains, Northwest China, is the largest volcanogenic BeU deposit in Asia. The deposit has three types of ores: U type, Be type, and BeU type. These ores either occur separately or co-exist but have cross-cutting relationships. Major ore bodies occur as fracture fillings along contact zones between the Yangzhuang granite porphyry and the underlying Devonian volcanic rocks. Three types of tourmaline (e.g., hosted in the granite porphyry, rhyolitic crystal tuff, and basalt) were identified based on their occurrences, and seven sub-type tourmalines were clarified according to their textures and chemical compositions. They all belong to the alkali schorl group and rich in F. The low Al concentration (< 6 apfu) and Fe + Mg > 3 apfu tourmaline hosted in the rhyolitic crystal tuff suggest the substitution at Z site by Fe3+. The tourmalines hosted in the granite porphyry are rich in Li, Be, U, Ca, Ti, Cr, Y, Zr, Nb, Mo, Sn, Sb, Ta, and Th, whereas the tourmalines hosted in the rhyolitic crystal tuff and the basalt are rich in Mg, Sc, V, Co, Cu, Ni, Ga, and Sr, respectively. The δ11B value ranges from −5.94‰ to −4.48‰, from −8.01‰ to −6.93‰, and from −8.31‰ to −4.94% in the tourmalines hosted in the granite porphyry, the rhyolitic crystal tuff and the basalt, respectively. These data indicate that the chemical components that have contributed to the growth of tourmaline were derived from granitic magmas essentially. The oscillating zonation and the apparent variations of trace element compositions (e.g., Fe/(Fe + Mg) and Na/(Na + Ca) ratios) and REE patterns of the tourmalines indicate the involvement of periodic fluids and significant fluid-rock interaction, which are responsible for the precipitation of sub-type tourmalines. It is proposed that the tourmalines at the Baiyanghe deposit are the products of fluids exsolved from either a deep-rooted crystallizing magma chamber or one of the younger intrusions of the hosting composite granitic pluton in addition to the crystallizing the Yangzhuang granitic magma. The chemical and boron isotopic composition of tourmaline can provide a fingerprint to decipher the magmatic-hydrothermal process during the cyclic UBe mineralization.

Chemical and boron isotopic compositions of tourmaline at the Dachang Sn-polymetallic ore district in South China: Constraints on the origin and evolution of hydrothermal fluids

The Dachang Sn-polymetallic ore district in South China is the second largest tin district in the world with a tin reserve of over one million tonnes. Zn-Cu skarn and stratiform, massive, and vein Sn-Pb-Zn ores are all present in this district. This has led to a debate as to whether the Sn orebodies were formed by Cretaceous magmatic-hydrothermal replacement or Devonian submarine exhalative-hydrothermal sedimentation. Here, we present a systematic investigation of the major, trace element, and boron isotopic compositions of different types of tourmaline in the Dachang ore district. Tourmaline disseminated in the Longxianggai granite and pegmatite veins belongs to the schorl series and has high contents of Li, Zn, and Ga. The δ11B value of primary magma of the Longxianggai granite is estimated to be about −13‰, close to the global average δ11B value (−11‰) for S-type granites. Tourmaline from quartz-tourmaline veins in the Longxianggai granite has similar chemical composition to the magmatic tourmaline and likely formed from hydrothermal fluids exsolved from the evolved granitic melt. The δ11B value of the initial hydrothermal fluids is also calculated to be about −13‰. Tourmalines from the skarn and sulfide ores in the Lamo deposit have higher Mg/(Mg+Fe) and lower Na/(Na+Ca) ratios and higher contents of Be, Ge, Sr, and Sn than magmatic tourmaline. These patterns likely reflect input of elements derived from the host Devonian limestone. The δ11B values of the hydrothermal fluids are estimated to be between −13 and −10‰, suggesting evolved magmatic-hydrothermal fluids related to the Longxianggai granite. Tourmalines from the stratiform and vein ores in the Changpo-Tongkeng deposit are extremely Mg-rich and mostly belong to the dravite series. They have high contents of Sc, V, Cr, Sr, and Sn and show positive Eu anomalies. The δ11B values of these B- and Sn-rich fluids are estimated to be between −15 and −10‰, suggesting that the fluids also have a magmatic-hydrothermal origin. These fluids are most likely derived from the same granitic magma source, but may have interacted with the Devonian volcanic rocks.

Boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites

The boron isotopic composition of tourmaline and hambergite (Be2BO3[OH,F]) from peraluminous (n = 12), peralkaline (n = 1), and peralkaline nepheline syenite (n = 16) pegmatites has been measured by secondary ion mass spectrometry, for which a new hambergite reference material was developed. The focus of this study is on nepheline syenite pegmatites from the Larvik Plutonic Complex (Norway) and one peralkaline pegmatite related to the nearby Eikeren-Skrim Complex (Norway), where we investigate the source of boron as being from magmatic vs. external fluids. Tourmaline-hambergite mineral pairs were also analysed from peraluminous pegmatite localities (Russia, Tajikistan, and Pakistan) to test for systematic B-isotope fractionation between these two minerals.Tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios (δ11B = −12.9to −1.0‰) associated with S-type granites, whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios (δ11B = −1.7 to 11.8‰), which we interpret is a result of heavy‑boron enrichment from external fluids. Our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite.The results suggest a slight but consistent partitioning of B-isotopes between tourmaline and hambergite, with Δ11B = δ11Btourmaline−δ11Bhambergite in the range of approximately −3‰ to −5‰. Boron is in trigonal coordination with oxygen in both of these mineral phases as verified by NMR. Single crystal XRD analyses of tourmaline and hambergite reveal consistent longer <B-O > distances of tourmaline relative to hambergite. We attribute the boron isotopic fractionation to the longer <B-O > bond-lengths in tourmaline compared with hambergite.

The nature and sources of ore-forming fluids in the Bhukia gold deposit, Western India: constraints from chemical and boron isotopic composition of tourmaline

Gold mineralization in the newly discovered Bhukia deposit in northwestern India is hosted in Proterozoic metamorphosed volcano-sedimentary rocks of the Aravalli-Delhi Belt. Three generations of tourmaline occurring in different textural settings are recognized in the host rocks of the deposit. Fine-grained texturally early tourmalines (Tur-I) precipitated during the gold-sulfide mineralization stage. They are dravitic in composition and occur parallel to the S1 tectonic foliation as fine-grained crystals in tourmaline-albite layers within quartz-albite rock and as scattered grains in calc-silicate rocks. Coarse-grained texturally later schorls (Tur-II) are also restricted to calc-silicate and quartz-albite rocks and characterized by sector zoning. Late-stage type III tourmaline (Tur-III), also of schorl composition, replaces Tur-II along fractures and margins. They are interpreted to have formed during a phase of ore remobilization. All tourmalines, especially the schorls, are strongly enriched in Li, Ga, Mn and Zn with Ga concentrations being the highest ever reported in natural tourmalines (up to 1380 ppm). The boron isotope composition is similar in all three tourmaline types with the consistently light δ11B (−10.4‰ to −7.2‰) indicative of a continental source for B. The chemical and B-isotopic composition of tourmaline is suggestive of the involvement of two fluids, a granitic-derived hydrothermal fluid and metapelite-derived metamorphic fluid. The δ11B variations in the tourmalines can be explained by mixing between 11B-poor granitic-derived hydrothermal fluid and 11B-rich metamorphic-hydrothermal fluid. While high V (2110–4247, avg. 3339 ppm) in the early dravitic tourmalines indicate mixing of granitic-derived hydrothermal fluid with pelite-derived metamorphic hydrothermal fluids during gold-sulfide mineralization, the enrichment of Li, Mn, Zn and Ta, and depletion of V in the later schorlitic tourmalines suggest increasing influence of granitic-derived hydrothermal fluids during ore remobilization.

Origin of the atypical Puy-les-Vignes W breccia pipe (Massif Central, France) constrained by trace element and boron isotopic composition of tourmaline

The Puy-les-Vignes W deposit is an atypical wolframite-bearing hydrothermal breccia pipe hosted in migmatitic biotite-sillimanite gneisses in the northwestern French Massif Central. The deposit is characterized by volumetrically important tourmaline alteration allowing to investigate the fluid evolution of the ore-forming hydrothermal system. Four generations of hydrothermal tourmaline (Tur 1–Tur 4) formed during pre-, syn-, and post-mineralization stages were identified based on detailed petrographic observations and were analyzed in situ for their chemical and boron isotopic compositions. At the grain scale, tourmaline commonly shows oscillatory zoning and dissolution textures resulting from a multi-stage crystallization in a fluid-dominated system. The different generations of hydrothermal tourmaline have dravite-schorl compositions and show similar major and trace element contents falling into the field of metamorphic rocks. High concentrations of V, Cr, Sr and low concentrations of Li, Sn in tourmaline suggest a metamorphic-dominated origin of these elements. The boron isotopic compositions of tourmaline range between −13.3‰ and −7.8‰ and cannot unambiguously distinguish between a magmatic and a metamorphic fluid origin. Our data indicate that the chemical and boron isotopic composition of tourmaline was dominantly controlled by high-temperature fluid-rock interactions between the metamorphic basement and boron-rich, reduced, and low-salinity hydrothermal fluids. Based on these results, we propose a fluid evolution scenario for the Puy-les-Vignes ore-forming hydrothermal system. Release of magmatic-hydrothermal fluids from an unexposed peraluminous leucogranite at ca. 324 Ma is proposed as the main mechanism responsible for early greisenization and formation of disseminated Tur 1. This episode was followed by massive tourmalinization and hydraulic brecciation of the overlying gneisses producing a tourmaline-rich (Tur 2) crackle breccia by interactions with boron-rich hydrothermal fluids. Fluid-assisted reopening and collapse of the former tourmalinite at ca. 318 Ma yielded a matrix-supported quartz-tourmaline breccia pipe and formation of wolframite-bearing quartz veins accompanied by deposition of Tur 3. Finally, post-ore hydraulic fracturing at ca. 300 Ma led to formation of tourmaline-rich microbreccias (Tur 4) possibly from metamorphic fluids.

Insights into B-Mg-Metasomatism at the Ranger U Deposit (NT, Australia) and Comparison with Canadian Unconformity-Related U Deposits

The Ranger deposit (Northern Territory, Australia) is one of the largest uranium deposits in the world. Uranium mineralisation occurs in crystalline basement rocks and is thought to belong to the unconformity-related category. In order to address the sources of magnesium and boron, and the temperature of the fluids related to boron and magnesium metasomatism that occurred shortly before and during the main uranium stage, in situ analyses of chlorite and tourmaline were carried out. The chemical composition of tourmaline shows an elevated X-site vacancy and a low Fetot/(Fetot + Mg) ratio typical of Mg-foitite. Uranium-related chlorite has relatively low Fe content (0.28–0.83 apfu) and high Mg content (3.08–3.84 apfu), with Si/Al = 1.08−1.22 and Mg/(Mg + Fetot) = 0.80−0.93 indicating a composition lying between the clinochlore and Mg-amesite fields. Chlorite composition indicates crystallisation temperature of 101–163 °C. The boron isotopic composition of tourmaline shows a range of δ11B values of ~1–9‰. A model is proposed involving two boron sources that contribute to a mixed isotopic signature: (i) evaporated seawater, which is typically enriched in magnesium and boron (δ11B ~ 40‰), and (ii) boron from the crystalline basement (δ11B ~ −30 to +10‰), which appears to be the dominant source. Collectively, the data indicate similar tourmaline chemistry but significant differences of tourmaline boron isotopic composition and chlorite chemistry between the Ranger deposit and some of the Canadian unconformity-related uranium deposits. However, lithogeochemical exploration approaches based on identification of boron- and magnesium-enriched zones may be usefully applied to uranium exploration in the Northern Territory.

Chemical and boron isotopic composition of tourmaline from the Conadong leucogranite-pegmatite system in South Tibet

Tourmalines occur widely within leucogranites and pegmatites that intrude the leucogranites in the south Tibetan plateau. Morphological and geochemical (elemental and boron-isotopic) studies on these tourmalines are very limited. Furthermore, the tourmaline genetic correlation between the leucogranite and the pegmatite is also unknown. In this contribution, two types of tourmaline occurrences have been identified in the Conadong area, south Tibet, including randomly disseminated tourmaline in leucogranite (GT type) and radial, dendritic or massive tourmaline aggregates in pegmatite dikes that intrude the leucogranite (PT type). We performed major element and boron isotopic analyses of tourmaline using electron microprobe (EMPA) and laser ablation multi-collector inductively coupled plasma mass spectrometry, respectively. The two types of tourmalines are enriched in Fe, Na and Al but relatively depleted in Ca and Mg elements, with compositions close to alkali group tourmaline and schorl. All the tourmalines are likely the products of equilibrium crystallization and their dominance of Na was controlled by the magmatic compositions of their host rocks. The GT type tourmalines show a narrow range of δ11B values between −9.78 ± 0.81‰ and − 8.53 ± 0.68‰ (with a mean of −8.91 ± 0.18‰); whereas the PT type tourmalines have significantly lower δ11B values than the GT type, ranging from −14.02 ± 0.85‰ to −11.83 ± 0.57‰ (with a mean of −13.31 ± 0.49‰). These data together with their morphology and petrography indicate that the Conadong granitic tourmalines were derived by the melts at the early magmatic stage, whereas the pegmatitic tourmalines precipitated from magmatic–hydrothermal fluids at the late magmatic stage. The variation in boron isotopic compositions (Δ11B = 4.4‰) between the PT and the GT type tourmalines is most likely correlated with their different melt/fluid source regions that had distinct Sr-Nd-B isotopic compositions.

Chemical and boron isotopic compositions of tourmaline in the Longtoushan hydrothermal gold deposit, Guangxi, China: implications for gold mineralization

The Longtoushan hydrothermal gold deposit is located in the southwestern region of the Dayaoshan Uplift. Tourmaline is widespread in the Longtoushan gold deposit and is mainly distributed in the rhyolite porphyry and associated cryptoexplosive breccia. The spatial distribution of tourmaline enrichment is similar to that of the gold orebody. Feldspar has been largely replaced by tourmaline in the rhyolite porphyry and cryptoexplosive breccia. Electron microprobe analysis revealed that tourmalines in the Longtoushan deposit belong mainly to the alkali group and partly to the X-vacant group; they mostly fell in the schorl-dravite series field. Two distinct sets of dominant substitutions were observed: MgFe−1 and Al□(NaR2+)−1, where R = Fe, Mg. In addition, minor substitutions include (CaMg)(NaAl)−1 and FeAl−1. The calculated δ11B value for the mineralizing fluids ranged from − 12.8 to − 9.7‰, which is typical of S-type granites, and boron-enriched fluids predominantly derived from rhyolitic melt. Part of the tourmaline from the rhyolite porphyry crystallized during the magmatic-hydrothermal stage, whereas most tourmalines from the deposit formed in the post-magmatic hydrothermal stage. The tourmalines were deposited from a relatively reduced and acidic fluid system, and the gold predominantly precipitated during the post-magmatic hydrothermal stage in the Longtoushan deposit.

Tourmaline geochemistry and boron isotopic variations as a guide to fluid evolution in the Qiman Tagh W–Sn belt, East Kunlun, China

The Qiman Tagh W–Sn belt lies in the westernmost section of the East Kunlun Orogen, NW China, and is associated with early Paleozoic monzogranites, tourmaline is present throughout this belt. In this paper we report chemical and boron isotopic compositions of tourmaline from wall rocks, monzogranites, and quartz veins within the belt, for studying the evolution of ore-forming fluids. Tourmaline crystals hosted in the monzogranite and wall rocks belong to the alkali group, while those hosted in quartz veins belong to both the alkali and X-site vacancy groups. Tourmaline in the walk rocks lies within the schorl–dravite series and becomes increasingly schorlitic in the monzogranite and quartz veins. Detrital tourmaline in the wall rocks is commonly both optically and chemically zoned, with cores being enriched in Mg compared with the rims. In the Al–Fe–Mg and Ca–Fe–Mg diagrams, tourmaline from the wall rocks plots in the fields of Al-saturated and Ca-poor metapelite, and extends into the field of Li-poor granites, while those from the monzogranite and quartz veins lie within the field of Li-poor granites. Compositional substitution is best represented by the MgFe−1, Al(NaR)−1, and AlO(Fe(OH))−1 exchange vectors. A wider range of δ11B values from −11.1‰ to −7.1‰ is observed in the wall-rock tourmaline crystals, the B isotopic values combining with elemental diagrams indicate a source of metasediments without marine evaporates for the wall rocks in the Qiman Tagh belt. The δ11B values of monzogranite-hosted tourmaline range from −10.7‰ and −9.2‰, corresponding to the continental crust sediments, and indicate a possible connection between the wall rocks and the monzogranite. The overlap in δ11B values between wall rocks and monzogranite implies that a transfer of δ11B values by anataxis with little isotopic fractionation between tourmaline and melts. Tourmaline crystals from quartz veins have δ11B values between −11.0‰ and −9.6‰, combining with the elemental diagrams and geological features, thus indicating a common granite-derived source for the quartz veins and little B isotopic fractionation occurred. Tourmalinite in the wall rocks was formed by metasomatism by a granite-derived hydrothermal fluid, as confirmed by the compositional and geological features. Therefore, we propose a single B-rich sedimentary source in the Qiman Tagh belt, and little boron isotopic fractionation occurred during systematic fluid evolution from the wall rocks, through monzogranite, to quartz veins and tourmalinite.

Geochemistry and boron isotopic compositions of tourmaline from the Paleoproterozoic amphibolites, NE China: Implications for the origin of borate deposit

The Paleoproterozoic metasediments and metavolcanics in the Jiao-Liao-Ji belt, northeastern China, host a large number of borate deposits. To trace the source of boron in these borate deposits, we carried out in situ chemical and boron isotopic studies on tourmalines of amphibolites from the Lieryu Formation of the Liaohe Group in the Jiao-Liao-Ji belt. The petrology and the chemical compositions of tourmalines of amphibolites show that these tourmalines formed by influx of B-rich fluids during different stages of metamorphism. The δ11B values of tourmaline continue to decline during the successive metamorphism. Tourmalines that formed during the prograde metamorphism show higher δ11B values (+9.52∼+10.18‰) than those formed during the peak metamorphism of amphibolite facies (+7.22∼+9.43‰). Subsequently, tourmalines with lower δ11B values formed during the retrograde metamorphism of greenschist facies, which have slightly different boron isotopic compositions in cores (+4.18∼+5.50‰) and rims (+2.66∼+4.66‰), mainly due to the temperature effect. The boron isotopic variations of tourmalines of amphibolites match well with our modeling study of boron isotope. The results suggest that tourmalines may be precipitated from fluids released from dehydration of marine sediments with relative high δ11B values (+9‰) during prograde metamorphism. In the same way, the borate deposits formed from the liberated B-rich fluids that interacted with the Mg-rich carbonates and/or silicates.

Chemical and boron isotopic composition of tourmaline from the Mariinsky emerald deposit, Central Urals, Russia

Tourmaline is abundant at the Mariinsky schist-hosted emerald deposit in the Central Urals, Russia, both in emerald-bearing phlogopite veins (type 1) and later, emerald-free pockets, lenses, and veinlets cutting the phlogopite veins (type 2). The Ca content in tourmaline is influenced by the host rocks (ultramafic and mafic rocks), associated minerals, and minerals crystallized before tourmaline (amphibole, fluorite, margarite). The Na concentration in tourmaline depends on the presence or absence of paragonite, and the association with micas also strongly influences the contents of Li, Zn, Ni, and Co in tourmaline. Type 1 tourmalines associated with phlogopite are relatively depleted in these elements, whereas type 2 tourmalines associated with margarite or paragonite are enriched. Some differences in isomorphic substitutions along with the trace element composition (Zn, V, Sr, Co, REE) may have value in exploration of emerald-bearing and emerald-free veins in schist-hosted emerald deposits. The δ11B values in tourmaline of all types fall in a narrow total range from −11.3 to −8.4‰. These values, combined with a mineralization temperature of 420–360 °C, yield an estimated δ11B fluid composition of −7.4 to −6.8‰ suggesting a mixed source of boron, likely dominated from the granitic rocks surrounding the emerald belt. The narrow range of B-isotope compositions in tourmaline from throughout the Mariinsky deposit suggests a well-mixed hydrothermal system.